@briechez: One of the Merriam Webster Online definitions of “colloquial” is “unacceptably informal,” which is what I was going for. You’re more likely to hear “could of” at a KKK rally than a MENSA meeting, where it would definitely be termed incorrect.

@defectiveuser
I’m a grammar nazi too, but it riles me when sources like englishplus.com flatly state that “could of does not exist.” It obviously does (at least here in the Ozarks), linguistically if not grammatically. In accurately transcribing dialogue, “could of” can be just as appropriate as “ain’t” under certain (very, very casual) circumstances.